Traditionally, computer systems are designed to be able to continuously run a fairly worst-case power load. Design according to such a continuous worst-case power load has never been much of a problem, because traditionally the individual components have had modest operating powers and the computer systems have had large power budgets so that the systems could sustain the load fairly naturally.
As the operating power consumptions of the individual components of computer system creep upwards, the power budgets of the computer systems have become tighter. It is now becoming a challenge to design a computer system to run a continuous worst-case workload while pursuing other goals, such as high computing power, compactness, quietness, better battery performance, etc. For example, portable computer systems, such as laptop computers, have a limited battery output capability; and thus a system designed to deal with a worst-case workload for a given battery output capability may limit the performance of the system because the worst-case workload may rarely occur.
Therefore, managing power is important to achieve both battery life and thermal design goals of the computer system or other data processing system, such as a cellular telephone. One common technique, for example, for managing the power of the central processing unit (“CPU”) is to dynamically adjust both the frequency of operation and the power supply voltage of the CPU core between multiple distinct states of the computer system. Typically, at a power operating point of the computer system there may be two components to power consumption, such as dynamic power and leakage power. Dynamic power represents the actual desired circuit operation. It is proportional to the number of clock transitions per second (frequency) and the square of the voltage. Leakage power represents the cost overhead of having the CPU powered at all. It is fixed for a given voltage, and normally rises exponentially with increasing voltage. Dynamically adjusting the frequency of operation, however, only manages the dynamic power and has no effect on the leakage power of the computer system.
Certain prior art systems included the ability to periodically cause the system to stop executing instructions by setting a system clock at zero frequency; however, these systems repeatedly performed this operation without regard to any measure of actual idle time in the system.